1. Field of the Invention
The present invention relates to novel ionic polymer device structures and novel methods of fabricating ionic polymer devices that can be configured as actuators, sensors, and transducers.
2. Description of the Related Art
Ionic polymer or ionomer composite material is one of the emerging classes of electroactive polymers and functional smart materials that can be made into soft bending actuators and sensors. The material was originally manufactured for fuel cell applications and its unique biomimetic sensing-actuating properties were not found until 1992. A typical ionic polymer actuator/sensor element comprises a thin polyelectrolyte ion-exchange polymer membrane in the middle as a dielectric layer and plated metal layers on two opposite surfaces of the ionomer membrane as electrodes. The ion-exchange polymer typically has a hydrophobic backbone and negatively charged hydrophilic functional groups (anion) as side chains. These side chains are associated with positively charged mobile cations. When the ion-exchange polymer absorbs a solvent, the interconnected solvent-containing cluster network is formed within the polymer matrix. While the anion are fixed to the polymer backbone (polymer matrix), the cations are free to move from cluster to cluster within the solvent upon electric stimulation. Conventional ionic polymer composite uses perfluorinated ion-exchange polymers as base polymers, such as a perfluoro-sulfonic polymer (Nafion®) and perfluoro-carboxylic polymer (Flemion®). These materials are soft and have a small mechanical stiffness.
When a potential is applied to the ionic polymer actuator, the unbound cations can move in and out of the clusters through the solvent and redistribute within the ionic polymer itself to form anode and cathode boundary layers. The change in electrostatic force and osmotic pressure, balanced by the elastic resistance, drives solvent into or out of the boundary layer clusters, and causes change in the volumes of interconnected clusters at this boundary-layer. This change in volume leads to the deformation or bending of the actuator. The charge distribution and the change in water uptake may be calculated by a coupled chemo-electro-mechanical formulation.
Ionic polymer materials offer significant advantages over conventional electromechanical materials and systems due to their compact sizes, light weight and the ability to be cut into any shape from the fabricated material. The fabricated device requires only modest operating voltage. The ionic polymer actuator can respond to small electric stimulus by generating large bending deformation, while the ionic polymer sensor responds to mechanical deformation (or vibration) by generating electrical signals. The sudden bent of the ionic polymer produces a small voltage (in the range of mV). In addition, the actuating/sensing function can be tailored by changing the micro-structure, the electrical input, the cation composition, and the solvent type and amount. The material is biocompatible and can be operated in various kinds of solvents. It may be developed to provide new, self-integrated material systems for biomedical and robotic applications.
One of many factors that can affect the coupled chemo-electro-mechanical responses of an ionic polymer based sensor/actuator is the electrode morphology and effective electrical capacitance. Traditional fabrication method for forming electrodes on an ionic polymer device involves first roughening and cleaning the surface of an already cured polymer membrane, allowing a substance capable of undergoing chemical reduction to be absorbed from the polymer surfaces, and reducing the absorbed substance to form electrodes. It normally requires repeated absorbing and reduction steps to allow more substance to diffuse into the ionic polymer membrane, and therefore a lengthy and expensive process. However, the diffusion of substance into a polymer membrane is still limited to less than about 20 microns from the membrane surface. Not only is the fabrication process expensive, the performance of the ionic polymer actuator/sensor is also affected by the diffusion limitation of the conductive material.